Transformers are electrical devices used to transfer electrical power from one circuit to another. Transformers are used extensively in the transmission and distribution of electrical power, both at the generating end and the consumer's end of the power distribution system. Such transformers include distribution transformers that convert high-voltage electricity to lower voltage levels acceptable for use for commercial and residential customers. These include network transformers that supply power to grid-type or radial secondary distribution systems in areas of high load density. These areas of high load density include, for example, underground, metropolitan vault applications, government, commercial, institutional and industrial facilities, and office towers and skyscrapers. Network transformers typically receive power at a higher distribution voltage and provide electric power at a lower voltage to a secondary network.
Transformers can be categorized in various ways, including the type of insulation (liquid immersed or dry-type), number of phases (single-phase or multi-phase), voltage level, or capacity. In addition, in the case of network transformers, transformers can be classified based on their type of installation. For example, “vault-type” network transformers are designed for installation in below-ground vaults, where occasional submersion may occur. On the other hand, “subway-type” network transformers are designed for installation in subsurface vaults, where frequent or continuous submerged operation is likely. Subway designs may also be used in vault-type applications.
Transformers are typically configured to include a core and conductors that are wound around the core so as to form at least two windings (or coils). These windings or coils are installed concentrically around a common core of magnetically suitable material such as iron and iron alloys and are electrically insulated from each other. The primary winding or coil receives energy from an alternating current (AC) source. The secondary winding receives energy by mutual inductance from the primary winding and delivers that energy to a load that is connected to the secondary winding. The core provides a circuit for the magnetic flux created by the alternating current flowing in the primary winding and which includes the current flow in the secondary winding. The core and windings are typically retained within an enclosure or tank for safety and to protect the core and coil assembly from damage. The tank also provides a clean environment, free of moisture. The tank is typically filled with an insulating fluid that provides electrical insulation value, while also serving to conduct heat from the core and coil assembly to the tank surface or cooling panels.
Although transformers are designed to operate efficiently at extreme temperatures, including relatively high temperatures, excessive heat is detrimental to transformer life. Transformers, similar to other electrical equipment, contain electrical insulation, which is used to prevent energized components or conductors from contacting or arcing over to other components, conductors, structural members or other internal circuitry. Heat degrades insulation, causing it to lose its ability to perform its intended insulative function. Additionally, the higher the temperatures experienced by the insulation, the shorter the service life of the insulation. When insulation fails, an internal fault or short circuit may occur, which can cause the equipment to fail and may lead to system outages. Transformer arcing faults result in the sudden generation of gases from oil vaporization and decomposition, which increases the pressure inside the transformer tank. Arcing faults may also arise from failures of other components including grounding switches, tap changers, bushings, ground connections, and electrical cable connections thereto.
Catastrophic rupture of a transformer can occur when the pressure generated by the gases exceeds the rupture pressure limit of the transformer tank or any component thereof. Although extremely rare, such ruptures can result in the release of flaming gases and liquids, which can pose a hazard to the surrounding area as well as pollute the environment. A catastrophic rupture can also cause expulsion of hardware and components from the transformer. Thus, it is critical that temperatures in the transformer be maintained at an acceptable level and/or that other steps are taken to minimize any risk that may result from such catastrophic failure.
As described herein, transformers generally contain an electrical insulator, which may, for example, be of a “dry-type” solid or gaseous dielectric or a liquid dielectric coolant to prevent excessive temperature rise and premature transformer failure. Most commonly, transformers are provided with a liquid coolant to dissipate the heat generated during normal transformer operation and to electrically insulate the transformer components. This liquid coolant is often referred to as a dielectric fluid or oil and is selected based on properties that affect its ability to function effectively and reliably, including, but not limited to, flash and fire point, heat capacity, viscosity over a range of temperatures, impulse breakdown strength, gassing tendency, and pour point. Examples of these coolants include, but are not limited to dimethyl silicone, mineral oils, hydrocarbon oils, synthetic hydrocarbon oils, paraffinic oils, naphthenic oils, ester and other plant-derived fluids, among others. Examples of transformers using a liquid coolant to dissipate heat are described, for example, in U.S. Pat. No. 6,726,857 to Goedde et al., U.S. Pat. No. 8,717,134 to Pintgen et al., and in U.S. Pat. Pub. No. 2010/0133284 to Green et al., the subject matter of each of which is herein incorporated by reference in its entirety.
Network transformers are designed for continuous use for a number of years and with minimal oversight. In many instances these network transformers are not routinely checked or maintained. In long-term usage, corrosion resistance is of great concern. Network transformers often sit in a network vault such as in a basement of a building or in a vault beneath a sidewalk or roadway that may be occasionally or routinely flooded. Corrosion has been cited as causing 80% or more of transformer failures on certain utility systems.
For example, the transformer system described in U.S. Pat. No. 8,717,134 to Pintgen uses a weakened weld 40 that is positioned in the lowest point of the transformer cooling panel, which is the most flood-prone and thus at the greatest risk of corrosion damage. As seen in FIGS. 1-3, as this weakened weld 40 degrades, the release of internal fluid cannot be controlled. Additionally, because corrosion is an electrochemical process, sharp corners, such as in the corner of the transformer cooling panel, concentrate the electrochemical stress which enhances the corrosion effect and increases the risk of the tank developing a leak and releasing fluid to the environment. Oil loss without a preceding electrical fault event will directly lead to electrical failure of the core and coil assembly and a possible fire. A further deficiency to this configuration is that in a catastrophic event, spacers 38 securing the cooling panel(s) 14 to the transformer tank 12 detach to create additional volume to increase the amount of gas that the tank 12 and cooling panel(s) 14 can withstand without rupturing. However, this additional volume resulting from the buildup of fluid prior to release, as shown in FIGS. 2 and 3, may cause the transformer system to become wedged in the containment vault, making removal and repair of the damaged system more difficult.
U.S. Pat. No. 8,884,732 to Johnson et al., the subject matter of which is herein incorporated by reference in its entirety, describes a dry-type network transformer having a core and coil assembly insulated by a combustion-inhibiting gas. The combustion-inhibiting gas and core and coil assembly are disposed within a hermetically sealed enclosure that is encapsulated by a polymer sealant. The combustion-inhibiting gas comprises air, an inert gas or a mixture of gases and is maintained at a prescribed temperature and pressure to prevent the operating temperature of the transformer from exceeding 220° C.
However, dry-type network transformers such as those described by Johnson are believed to be unable to adequately contain the energy released during an internal arc fault event.
Thus, while various methods have been proposed for improving the pressure containment capabilities of a transformer tank, additional improvement means are still desired to provide a transformer tank that is able to adequately contain extreme pressures of gases therein. In addition, it is also desirable to provide an improved means of selectively and preferentially venting these gases and fluid from the transformer tank to prevent the tank from rupturing in a catastrophic manner under extreme electric fault energy conditions that exceed the containment pressure limit of the improved transformer design. Finally, it is also desirable to provide an improved pressure containment system that does not cause significant distortion of the outer dimensions of the transformer tank system that would cause the tank to become wedged in the containment vault.